Stereoselective epoxidation

Keywords epoxide, stereoselective, addition, ring opening, gas-solid reaction... 

How would you attempt to transform this allylic alcohol into both diastereoisomers of the epoxide stereoselectively You are not expected to estimate the degree of success. 

Allylic ethers are epoxidized stereoselectively to predominantly syn products with peroxytrifluoroacetic acid in dichloromethane. In tetrahy-drofuran, anti epoxidation prevails. Also, m-chloroperoxybenzoic acid in dichloromethane gives a higher proportion of anti-addition products (equation 333) [287. ... 

We shall discuss the conversion of 35 into 36 later in the chapter. Elimination and epoxidation stereoselectively produces the epoxide 37 on the outside (ejto-face) of the folded alkene. Nucleophilic opening with PhSe gives the iran.v-diaxial product (chapter 21). 

Henbest showed that in the absence of severe steric interference, allylic cyclohexenols are epoxidized stereoselectively by organic peroxy acids to furnish c/s-epoxy alcohols a large number of c/s-epoxy alcohols have been prepared by epoxidizing aUylic cyclohexenols. A mixture (5 1) of labile bisaUylic alcohols (19) and (20) was reacted with m-CPBA (eq 5) from the reaction mixture diepoxide (21) was isolated as a single isomer. Epoxidation of (Z)-cyclooct-2-en- l-ol (22) furnishes exclusively (99.8%) the trans-epoxide (23) (eq 6). Similar observations have been made subsequently. This result, as well as the stereoselectivity observed during the epoxidation of other allylic alcohols, both cyclic and acylic, has been rationalized on the basis of transition state models. ... 

As indicated in Scheme 8, preparing the test substrate for this novel reaction cascade, diol 57, did not prove overly challenging, although one major surprise was encountered in the course of its construction. The first steps of the sequence sought to install a C11-C12 epoxide stereoselectively (as expressed in 56), since... 

The stereochemistry of molybdenum catalyzed epoxidation of olefins has been studied in some detail. Early work [375] indicated that ca-olefins give cn-epoxides and trans-olefins give trans-epoxides stereoselectively, equations (234) and (235). 

Oxidation of cyclohexene by a peroxyacid (Section 11.8Q gives an epoxide. Stereoselective nucleophilic attack by azide ion anti to the leaving oxygen of the epoxide ring (Section 11.9B) followed by reduction of the azide with lithium aluminum hydride gives racemic fra s-2-aminocyclohexanol. 

The proposed mechanism for this transformation is closely related to that depicted in Scheme 40.47 above, except that the intermediate iminium ion derived from the primary amine catalyst 32 is epoxidated stereoselectively [60] and, after hydrolysis of the iminium, an acid-induced diastereoselective semipinacol rearrangement affords the spirocychc hydroxy-diketone product (Scheme 40.53). 

A diphenylpolystyryldiethoxyphosphorane (24), prepared from a DF =1.0 PS-diphenylphosphine and diethyl peroxide, was more stable and easier to handle than the diethoxytriphenylphos-phorane. The polymeric reagent cyclodehydrated diols to three-, five- and six-membered ethers in yields as high as the solution reagent, and converted 1,2-diols to epoxides stereoselectively. For example, S( + )-phenylethane-l,2-diol gives 68% enantiomeric excess (ee) S(4-)-styrene oxide as shown in equation (8), whereas only the racemic product was obtained from the soluble reagent. 

Lucero MJ, Houk KN. Torsional control of epoxidation stereoselectivity in 1,2-dihydronaphthalenes. Transition state modeling with semiempirical quantum mechanics. J. Org. Chem. 1998 63 6973-6977. 

For some oxiranyllithium species, which we have discussed in Chap. 2, two stereoisomers can exist. In such cases, an oxiranyllithium species has another side reaction that needs attention Isomerization caused by inversion of configuration at its carbon atom bound to lithium in addition to its decomposition involving ring opening. To synthesize a substituted epoxide stereoselectively, we need to suppress such isomerization and allow only one isomer to selectively react with an electrophile. For this purpose, high-resolution reaction time control using a flow microreactor system is effective. 

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